Optical Differential-Phase-Shift-Keyed (DPSK) demodulators have traditionally been formed by using the well known Mach-Zehnder interferometer. This device has essentially two “arms” that form distinct optical paths that are used to detect phase shifts in the optical DPSK signal. The Mach-Zehnder interferometer requires a physical length that is determined by the data rate of the communication link with which it is being used. The two arms of the Mach-Zehnder interferometer must maintain a differential optical path length equal to the distance that light travels in one bit period. As a result, it is highly sensitive to temperature and other environmental factors that cause variations in the physical dimensions of the optical components that it uses.
To address the undesirable influence that thermal factors have on the performance of present day Mach-Zehnder interferometers that are used in DPSK optical demodulator applications, these devices have typically employed a control loop to regulate the path length of one of the optical paths. As will be appreciated, this adds significantly to the complexity and cost of the interferometer.
Other apparatuses and methods for demodulating a DPSK optical signal have involved optical filter discriminator approaches that attempt to detect the phase shift changes in the bit pattern of the optical DPSK signal. However, such approaches typically only use the transmitted wavefront portion of the input optical signal, and not the larger reflected energy provided by the reflected wavefront component of the signal. Such approaches further typically do not attempt to make use of both the transmitted and reflected energy components of the optical DPSK signal. Using both the transmitted and reflected energy 1) increases the received signal, and 2) improves the signal-to-noise ratio, since common-mode noise of the dual detectors is cancelled out. Time alignment of the transmitted and received pulses is less difficult than alignment of the two arms of a Mach-Zehnder demodulator.